1. Technical Field
The present invention relates to an endoscopic objective lens and an endoscope.
2. Background Art
Heretofore, endoscopes have been used in the field of medicine to observe, for example, stomachs and esophagi. A peroral endoscope which is to be inserted from a mouth may sometimes give distress of nausea or discomfort to a patient due to touching of the endoscope to the root of the tongue or on the inner throat when the endoscope is inserted into the patient. In recent years, downsized and diameter-reduced endoscopes, including a nasal endoscope downsized enough to be inserted from a nose, have been used in order to alleviate such distress. When considering downsizing and diameter reduction of an endoscope, it is necessary to downsize a distal end portion of the insertion section. The distal end portion includes an observation objective lens, an illumination section, a treatment tool insertion opening, an air and water supply nozzle for cleaning the surface of the objective lens, and the like.
Among them, improvements have been made with respect to the objective lens, including reductions in the number of lenses, in overall length and in lens diameter for the downsizing. For example, an objective lens having a lens configuration constituted by only one negative lens on the object side of the stop is proposed for the downsizing of the distal end portion. Further, an objective lens having a lens configuration in which the object side of the stop is constituted by only one cemented lens is proposed in Japanese Unexamined Patent Publication No. 8 (1996)-122634 and Japanese Patent No. 4373819.